Various radiation detectors are known in the art, in particular U.S. Pat. No. 3,952,196, issued Apr. 20, 1976, and owned by the assignee of the present invention, discloses a device for determining whether the optical surfaces through which radiation must travel from a potential source of radiation to a radiation detector are free from radiation absorbing material or radiation-blocking material. The device disclosed therein includes an enclosure for commonly housing both the auxiliary light source and the radiation-detector tube, while internally isolating the same from each other, thereby preventing radiation transmission within the housing from the light source to the detector. The patent also discloses a radiation path from the light source outwardly from the housing to a reflective surface or surfaces external of the housing, which surfaces reflect at least some of the radiation back to the detector through the same optical surfaces that other external radiation passes. The patent contemplates external reflective surfaces which either form a part of the outside housing structure, or are remotely located therefrom.
Thus, the prior art device described above, provides a self-checking feature for radiation-detection devices in many applications. However, this device suffers disadvantages which limit its usefulness in particular applications. For example, when the radiation-detection apparatus is placed in a corrosive atmosphere, such as an atmosphere laden with chemically corrosive vapors, the external reflective surfaces tend to suffer therefrom, causing a degradation of their light reflectivity. As a result thereof, the degradation of the reflecting surfaces causes false fault indications and/or indication that the radiation-detection device is inoperative when in fact it continues to function normally in all respects except its self-checking features.
A further radiation-detecting device as seen in U.S. Pat. No. 4,405,234, issued Sept. 20, 1983, and also owned by the assignee of the present invention, discloses a device that eliminates the need for an external reflective surface, and therefore, is operable in corrosive environments. This device takes advantage of the refractive properties of light and employs a beveled glass window element through which light from the auxiliary light source passes and is reflected off the exterior interface surface thereof, to a mirror positioned across the window opposite from the auxiliary light source. The light is then reflected off the mirror toward the radiation detector. However, this device suffers from the disadvantage of the cost involved in the manufacture of the beveled glass window element, which is expensive. Also, the light path requires the precise alignment of the auxiliary light source, window element, mirror and radiation detector.
Accordingly, it would be desirable to provide for a radiation-detection apparatus having a self-contained checking capability, operable in a corrosive or contaminated environment, and to do so at substantially reduced cost.